Electronic deionization (EDI) is an established method for purification of water by removing ionic solutes. EDI uses filter units, hereafter referred to as cells, which have a membrane. EDI uses electricity in addition to a membrane to filter material including solutes. The membranes of cells can be regenerated electrically and by reverse flushing and are therefore regarded as re-usable. The number of regeneration cycles of cell may be limited because of gradual permanent degradation of the membrane of the cell.
Ionic solutes may be expressed as total dissolved solids (TDS) in parts per million (PPM). As indicated above, an EDI cell may be capable of 85% to 95% TDS reduction. Higher levels of purification can be achieved with three or more EDI cells connected in series to achieve a ratio of TDS in the input water to that in the output water as shown in the table below. For example, water containing 1000 ppm TDS may have the TDS reduced at each stage of a series-connected cells as follows.
1000 TDS×90% Stage 1=900 TDS Reduction | out=100 ppm
100 TDS×90% Stage 2=90 TDS Reduction | out=10 ppm
10 TDS×90% Stage 3=9 TDS Reduction | out=1 ppm
1.0 ppm remaining from 1000 TDS starting concentration
When an EDI cell is actively removing ions, membranes of the cell are charged with electricity creating anionic and cationic bonding sites. EDI cells require regeneration after a period of time. During regeneration, the polarity on the membranes is reversed and water forced in a reverse direction to push the ions out of the bonding sites and flush the captured ions out of the cell.
In EDI technology, a membrane, such as a re-usable membrane, has a finite service life. In general, the service life is a function of the number of regeneration cycles. During regeneration, the membrane is exposed to concentrated solutes. Progressive scaling of calcium carbonate and other solutes can occur during the regeneration cycle. As scale forms, the membrane becomes permanently degraded, ultimately requiring replacement.